The hydrogenation of a series of linear and cyclic diolefins diluted in decalin, at conditions (<= 250 degrees C, 3.4 MPa H-2 and LHSV of 1-2 h(-1)) chosen to mimic the hydrogenation of a Canadian oil sands naphtha (similar to 20 wt% olefins and 4-5 wt% diolefins) over a NiMoS/Al2O3 catalyst, is reported. The yield of dimers was minimal on fresh NiMoS/Al2O3 catalyst but increased significantly on spent NiMoS/Al2O3 catalyst recovered from a commercial hydrotreater. Dimer yield was also strongly dependent on the diolefin molecular structure. High dimer yields were obtained from molecules with conjugated double bonds, provided the double bond was not sterically hindered. Addition of 1-octene did not impact the dimer yield and 4-methylstyrene had the highest oligomer yield compared to all other diolefins examined. The results are consistent with the formation of resonance stabilized allylic carbocations being key to dimer formation. Increased concentration of the diolefin in the feed resulted in increased dimer yield and gum content of the product oil, providing a direct link between the presence of conjugated diolefins, their dimerization and gum formation that is the likely cause of catalyst activity loss and fouling of the catalyst bed.